During flight tests, that maneuver is known as a symetrical pushover. Main reason is to check compliance with FAR regarding pitch stability and control, as well as structural integrity.
On a personal note, did that once while inputing a maximum nose down hardover from the autopilot at the zero-G point. Not recommended! Quite the negative G-load! Nothing bent or broke, but a quick way to lose your breakfast, as most of the aircrew did, including myself.
The opposite maneuver was the symetrical pull-up. Always a good positive G-load there during the pull-up.